Structure, surface reactivity and physico-chemical degradation of fluoride containing phospho-silicate glasses

We report on the structure, apatite-forming ability and physicochemical degradation of glasses along the fluorapatite [FA; Ca5(PO4)3F]–diopside (Di; CaMgSi2O6) join. A series of glasses with varying FA/Di ratio have been synthesised by melt-quenching technique. The amorphous glasses could be obtained only for compositions up to 40 wt.% of FA. The detailed structural analysis of the glasses has been made by infrared spectroscopy (FTIR), Raman spectroscopy and magic angle spinning-nuclear magnetic resonance spectroscopy (MAS-NMR). Silicon was predominantly present as Q2 (Si) species while phosphorus was found in an orthophosphate type environment in all the investigated glasses. The apatite forming ability of glasses was investigated by immersion of the glass powders in simulated body fluid (SBF) for times varying between 1 h–28 days. An extensive precipitation of calcite (CaCO3) after immersion in SBF was found in all the glasses, which considerably masked the formation of hydroxyapatite [HA; Ca5(PO4)3OH]. The possible mechanism favouring the formation of calcite instead of HA has been explained on the basis of experimental results obtained for the structure of the glasses, leaching profile of glass powders in SBF solution and pH variation in SBF solution. Furthermore, the physico-chemical degradation of the glasses has been studied in accordance with ISO 10993-14 “Biological evaluation of medical devices – Part 14: Identification and quantification of degradation products from ceramics” in Tris HCl and citric acid buffer. All the FA containing glasses exhibited a weight gain (instead of weight loss) after immersion in citric acid buffer due to the formation of different crystalline products.

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